Though the toxicity of organic and inorganic mercury compounds is well established, as is the ability of many bacteria to transform these compounds in vitro to either more or less toxic forms, nothing is known about the ability of the intestinal bacterial flora to alter the toxicity of orally administered mercury compounds. We have established that it is possible (though only through administration of massive doses of HgCl2) to alter the intestinal flora of mice to contain a high percentage of mercury transforming bacteria. We have also found in a pilot study that hospitalized humans (obviously not dosed with HgCl2) frequently have an intestinal flora which is predominantly mercury-transforming bacteria. The work proposed herein is designed to devise alternative methods (less toxic than HgCl2 administration) of increasing the numbers of mercury transforming bacteria in the mouse intestinal flora, and to examine, in the mouse model system whether such a flora is effective in altering the toxicity of orally administered inorganic and organic mercury compounds. We also plan to complete the baseline data for the incidence and percentage of such a mercury-transforming flora in both hospitalized and non-hospitalized persons in order to assess those genetic, dietary, therapeutic, and environmental factors which lead to the establishment of such a flora in humans.